This invention relates to open-cell polyurethane foams and their manufacture.
The dimensional stability of a polyurethane foam can be an important consideration in a number of applications. In the case of predominantly open-cell polyurethane foams, the open cells enable the foam to readily breathe ensuring quick pressure and cell gas equilibration without the changes in cell size that may cause dimensional distortion in some closed-cell polyurethane foams. This dimensional distortion is particularly pronounced in closed-cell foams that are blown with water.
One current method of opening the cells of water-blown polyurethane foams is through the use of cell destabilizers, typically polydimethylsiloxanes (silicone oils). The problem with this approach is that cell wall opening tends to occur early in the foaming reaction thus destabilizing the foam. This can be overcome through the use of nonlinear silicone surfactants (polydimethylsiloxane backbones with pendant polyether chains) but the processing window that allows for the formation of a stable foam with open cell walls is often very narrow.
Another approach is to use linear block copolymers of polydimethylsiloxanes and polyethers [(polyether)(polydimethylsiloxane)]n. These (AB)n copolymers, in which A represents the polyether block, B represents the polydimethylsiloxane block and n represents the number of repeating units, stabilize gas entrainment and reduce bubble coalescence during the foaming process until late in the reaction when they are able to open holes in the cell walls. Typically, the minimum useful number average molecular weight required for these (AB)n copolymers is greater than 30,000 and preferably from 65,000 to 100,000. Below this molecular weight, an (AB)n copolymer will likely fail to adequately stabilize a polyurethane foam. A problem with (AB)n copolymers that stabilize the foaming reaction and open the cell walls is that they tend to form hydrogels in the presence of water. This can limit their usefulness, especially when fully formulated resins containing both the (AB)n surfactant and water must remain stable for relatively extended periods of time prior to use. A second problem with (AB)n copolymers is that it can be difficult to control their molecular weight during manufacture.